US20090295952A1 - Gain matching for electron multiplication imager - Google Patents
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- US20090295952A1 US20090295952A1 US12/128,890 US12889008A US2009295952A1 US 20090295952 A1 US20090295952 A1 US 20090295952A1 US 12889008 A US12889008 A US 12889008A US 2009295952 A1 US2009295952 A1 US 2009295952A1
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- 238000000034 method Methods 0.000 claims abstract description 23
- 238000012546 transfer Methods 0.000 claims abstract description 15
- 239000007943 implant Substances 0.000 claims abstract description 6
- 238000003384 imaging method Methods 0.000 claims description 14
- 230000004888 barrier function Effects 0.000 claims description 3
- 230000000750 progressive effect Effects 0.000 claims description 3
- 238000010586 diagram Methods 0.000 description 8
- 238000009792 diffusion process Methods 0.000 description 6
- 238000006243 chemical reaction Methods 0.000 description 2
- 238000013461 design Methods 0.000 description 2
- 238000009966 trimming Methods 0.000 description 2
- 238000003491 array Methods 0.000 description 1
- 239000003990 capacitor Substances 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000005684 electric field Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
- H01L27/144—Devices controlled by radiation
- H01L27/146—Imager structures
- H01L27/14601—Structural or functional details thereof
- H01L27/14609—Pixel-elements with integrated switching, control, storage or amplification elements
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
- H04N25/60—Noise processing, e.g. detecting, correcting, reducing or removing noise
- H04N25/67—Noise processing, e.g. detecting, correcting, reducing or removing noise applied to fixed-pattern noise, e.g. non-uniformity of response
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
- H01L27/144—Devices controlled by radiation
- H01L27/146—Imager structures
- H01L27/14643—Photodiode arrays; MOS imagers
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
- H04N25/70—SSIS architectures; Circuits associated therewith
Definitions
- the present invention relates generally to imaging systems, and more particularly to equalization of gain in the output of an array of imaging pixels which employ electron multiplication (impact ionization).
- CCD charge-coupled device
- charge is then transferred back from the temporary holding gate 10 to the accumulating gate 4 which is now biased as an impact ionization gate. This is accomplished by pulsing the holding gate 10 to a lower potential and transferring the charge through the intermediate gate 8 to the impact ionization region. For further gain, this procedure is repeated multiple times, i.e. 100 to 500 times, to build up charge.
- the gain per impact ionization transfer is roughly 1.015 ⁇ .
- the gain after N impact ionization transfers is roughly (1.015) N . For N equal to 400, the resulting gain is about 386.
- FIGS. 2A and 2B plan views of the Tower et al. EMCMOS device layout and architecture are depicted.
- Charge collection, storage, and electron multiplication (EM) regions are incorporated into a single pixel 30 formed monolithically as an integrated circuit.
- the pixel 30 includes electron multiplication (EM) gain regions 32 , 34 whose accumulated charge circulates around an enclosed track or circulating register called the EM gain register 36 .
- EM electron multiplication
- the Tower et al. device can comprise one or more impact ionization gain stages with implants to achieve charge transfer directionality.
- a readout structure 54 comprising a number of sub-structures are fabricated in the pixel 30 nested within the EM gain register 36 .
- the light sensitive area, which creates electrons in proportion to the radiant energy incident on the pixel 30 can be an optical-to-charge conversion device such as a pinned photodiode (PPD) 56 as shown, a photogate, etc.
- PPD pinned photodiode
- the PPD 56 is connected to and releases the accumulated charge to the EM gain register 36 by means of a PPD transfer gate (TR 1 ) 58 .
- TR 1 PPD transfer gate
- a floating diffusion sense node 60 for receiving amplified charge from the EM gain register 36 and for converting the charge to a voltage is also connected to the EM gain register 36 by means of a floating diffusion transfer gate (TR 2 ) 62 .
- the readout circuitry 54 includes a row select gate 64 , a PPD reset gate 66 , a source follower transistor 68 , and a source follower reset gate 70 .
- Power is supplied to the pixel 30 by means of power rail VDD 72 .
- the pixels 30 can be manufactured using a CMOS process, preferably a PPD CMOS process.
- FIGS. 3A-3C a schematic cross-section of one stage (4 gates) of the (EM) gain register 36 and accompanying applied potential diagrams of the pixel 30 is depicted.
- Each EM gain stage includes four gates: a first DC gate 74 , a phase 1 clock gate 76 , a second DC gate 78 , and a phase 2 clock gate 80 which is employed to control the electron multiplication function.
- the clocking of the EM gain register 36 is done with two clock phases, as shown in FIG. 4A . During a first clock period shown in FIG.
- the phase 1 clock gate 76 has a first potential applied to it to hold the accumulate charge (electrons) in a charge packet 82 in a potential well 84 below the phase 1 clock gate 76 . Meanwhile, a larger potential is applied to the phase 2 clock gate 80 to create a potential well 86 , which is initially devoid of charge. Barrier regions 88 , 90 are created below the DC gates 74 , 78 , respectively, to assure proper direction of charge transfer between the clock gates 76 , 80 and between the (EM) gain regions 32 , 34 by the application of appropriate low DC voltage levels relative to the high voltages applied to the clock gates 76 , 80 when they are in their high (on) state.
- EM electrostatic electrostatic voltage
- the potential of the phase 1 clock gate 76 is changed to about 0 volts which is lower than the potential applied to the DC gates 74 , 78 . Electrons that have accumulated below the phase 1 gate clock 76 now “spill” over into the well 86 and can undergo impact ionization at the interface 94 between the second DC gate 78 and the phase 2 clock gate 80 . As a result, the electron charge packet 82 transfers to the well 86 . Proper directionality is achieved by optimized implants.
- the electrons in a charge packet 82 are made to circulate a predetermined number of times through the EM gain regions 32 , 34 in the pixel 30 .
- the probability of impact ionization, and thus the mean gain per stage g for each of the EM gain regions 32 , 34 is low, the number of times that the charge packet 82 pass around the EM gain registers 36 and through EM gain regions 32 , 34 in the pixel 30 , designated as N, can be high.
- FIGS. 4A-4D illustrate the operation of the pixel 30 of FIGS. 2A and 2B .
- a charge packet 82 accumulated in the pinned photodiode (PPD) 56 is loaded into the EM gain register 36 via the PPD transfer gate 58 (TR 1 ).
- the charge packet 82 circulates around the EM gain register 36 through the EM gain regions 32 , 34 .
- the charge packet 82 circulating around EM gain register 36 accumulates nominally about 400 transfers through the EM gain regions 32 , 34 (200 cycles).
- a third time period FIG.
- the amplified charge packet 82 is transferred to the floating diffusion sense node 60 by means of the floating diffusion transfer gate 62 where the charge packet is converted to a voltage, the floating diffusion sense node 60 acting like a capacitor.
- the floating diffusion sense node 60 and the PPD 56 are reset (global reset) via the PPD reset gate 66 . Then the sequence is repeated for the next frame.
- Each pixel comprises one or more impact ionization gain stages with implants to achieve charge transfer directionality.
- the invention is built upon the fact that the gain in the pixel architecture chosen is determined by the voltage difference between the DC gate and the high level of the EM clocked gate.
- a means is provided that permits the DC gate level to be adjusted individually when the selected row of pixels is being clocked to produce impact ionization gain.
- each pixel EM gain stage comprising a phase 1 clocked gate, an EM clocked gate, and DC gates formed between the phase 1 clocked gates and the EM clocked gates, comprising the steps (a) applying initial voltages to each of the DC gates and the EM clocked gates of at least two pixels of a plurality of pixels; (b) clocking phase 1 clock gates and an EM clock gates associated with the at least two pixels of the plurality of pixels a predetermined number of times to achieve pixel intensity values after impact ionization gain; and (c) selectively adjusting the difference in voltage between the DC gate and corresponding EM clocked gate of the at least two pixels of the plurality of pixels until the difference between the resulting pixel intensity values and the average pixel intensity value needed to produce a desired image is below a predetermined threshold.
- the method can further comprise the step of, before step (b), exposing the plurality of pixels to light reflected from a flat field; wherein step (c) further comprises the steps of: (d) selectively adjusting voltages applied to the DC gates associated with the at least two pixels of the plurality of pixels such that substantially all of the pixels are projected to have the same gain; and (e) repeating steps (b) and (c) until the difference between the resulting pixel intensity values and the average pixel intensity value needed to produce a desired flat field image is below a predetermined threshold.
- the array of EM pixels can be arranged in a 2-dimensions array of rows and columns, each row including a common phase 1 clock gate bus and a common EM clock gate bus, each column including a common DC gate bus, and wherein step (c) further comprises the steps of (f) selecting a row of pixels; (g) applying a first set of DC voltages to the DC gate buses; (h) clocking a phase 1 clock gate bus and an EM clock gate bus associated with the row of pixels; and (i) holding the phase 1 clock gate buses and EM clock gate buses of the other rows of pixels at predetermined constant DC voltages so as to be inactive.
- Step (c) can further comprise the steps of (j) selecting another row of pixels; (k) applying a second set of DC voltages to each of the DC gate buses; (l) clocking a phase 1 clock gate bus and an EM clock gate bus associated with the another the row of pixels; (m) holding the phase 1 clock gate buses and EM clock gate buses of the other rows of pixels at predetermined constant DC voltages so as to be inactive; and (n) repeating steps (j)-(m) until substantially all rows in the plurality of pixels have achieved the same pixel intensity value after EM gain.
- Each row can be associated with a row selection bus, wherein step (d) further comprises the steps of (o) selecting a row of pixels via the row selection bus; (p) reading a set of output voltage values of each of the columns; (q) storing the set of output voltage values in a memory; (r) selecting an additional row of pixels; (s) repeating steps (o)-(r) until substantially all of the rows in the plurality of pixels have achieved the substantially the same gain.
- the method can further comprise the steps of recalling all of the stored voltage values; performing a calculation to determine whether a flat field image has been achieved; calculating a plurality of DC voltages to be applied to corresponding ones of the DC gates in the array of pixels to achieve substantially the same gain; and storing values corresponding to the plurality of DC voltages in the memory.
- FIGS. 1A and 1B depicts schematic cross-section of CCD gates and accompanying applied potential diagrams which illustrate the principle of impact ionization resulting in electron multiplication as outlined in the Gager paper of the prior art;
- FIGS. 2A and 2B are plan views of a Tower et al. EMCMOS pixel layout and architecture of the prior art
- FIGS. 3A-3C depict a schematic cross-section of the electron multiplication gates and accompanying applied potential diagrams of the pixel of FIGS. 2A and 2B ;
- FIGS. 4A-4D illustrate the operation of the pixel of FIGS. 2A and 2B ;
- FIG. 5 is a block diagram of an arrangement of a two-dimensional array of electron multiplication (EM) pixels employing a calibration procedure according to an embodiment of the present invention
- FIGS. 6A and 6B depict a timing diagram showing clocking conditions of the EM clock bus lines and the phase 1 clock bus lines of the imaging array of FIG. 5 during calibration time;
- FIG. 7 is a block diagram of the two dimensional array of EM pixels of FIG. 5 incorporated into a calibration fixture.
- FIG. 8 is a flow chart illustrating steps of the calibration procedure of the present invention.
- the gain of an EM gain stage 36 of a pixel 30 for the Tower et al. device is proportional to the electric field associated with the potential well 86 .
- the impact ionization gain is set by the difference between the voltage applied to the phase 2 (EM) clock gate 80 and the voltage applied to the DC gate 78 .
- the voltage differences between corresponding phase 2 (EM) clock gates 80 and DC gates 78 can be trimmed.
- FIG. 5 shows an arrangement of a two-dimensional array 100 of electron multiplication (EM) pixels 101 employing a calibration procedure according to an embodiment of the present invention.
- An EM pixel 101 is located at each of the intersections of a plurality of horizontal EM clock bus lines 102 a - 102 n, phase 1 clock bus lines 104 a - 104 n, and vertical DC gate bus lines 106 a - 106 n.
- Each row of tie pixels 101 can have a common EM clock bus line, e.g., 102 a, (labeled N, N+1, N+2, etc.) and phase 1 clock bus, e.g., 104 a, (labeled N, N+1, N+2, etc.).
- Each column of pixels 101 can have a common DC gate bus line, e.g., 104 a, (labeled (M, M+1, M+2, etc.).
- Each of the column DC gate bus lines 104 a - 104 n can be independently controlled. If only one row (e.g., N) is clocked at one time, each of the pixels 101 in that row can have its gain adjusted pixel by pixel, by individual adjustment of the separate column DC gate bus lines 104 a - 104 n.
- the pixels 101 are provided with horizontal row selection lines 108 a - 108 n a vertical column bus selection lines 110 a - 110 n, and a plurality of column bus amplifiers 112 a - 112 n.
- a timing diagram shows clocking conditions of the EM clock bus lines 102 a - 102 n and the phase 1 clock bus lines 104 a - 104 n during calibration time (which can also mirror the relative clocking times during normal operation as shown in FIGS. 3B and 3C ).
- the voltage levels of an inactive row, e.g., row N+1, are held at constant DC levels such that no gain occurs in the pixels of that row.
- the EM clock bus line 102 b for row N+1 is held “low,” while the phase 1 clock bus line 102 b for row N+1 is held “high.”
- the EM clock bus line 102 a is also held “low,” while the phase 1 clock bus line 104 a for row N is held “high.”
- the EM clock bus line 102 a input is set to high, but because of capacitance a finite time is needed before the EM clock bus line voltage settles at about time t 1 .
- both the phase 1 clock bus line 104 a is still held at a “high” level in order to allow for the voltage across the EM clock bus line 102 a to settle.
- the phase 1 clock bus line 104 a is set to “low,” which allows charge that has accumulated under a phase 1 gate to “spill” over into a charge well formed by the application of the high EM clock gate voltage during time interval T 4 and thereby acquiring gain through impact ionization.
- the phase 1 clock gate voltage is set back to high, but needs a finite amount of time to settle.
- the EM clock bus line 102 a is set back to “low”, but takes a finite time to settle.
- the calibration fixture includes a camera 118 with focusing optics 120 , at least one a processor 124 , and a memory 126 .
- the imaging array 100 and the calibration fixture 116 can be incorporated entirely into a functioning camera or can be incorporated into stand-alone factory test equipment.
- the vertical DC gate bus lines 106 a - 106 n are set to a predetermined nominal DC voltage stored in the memory 126 for all of the pixels 101 .
- light 121 reflected from a flat field image 122 is focused by the focusing optics 120 of the camera 118 onto the imaging array 100 .
- a first active row is clocked for a predetermined number of clock cycles (to achieve a desired average pixel intensity value) with the waveforms discussed in FIG.
- step 132 if this is not the last row in the imaging array 100 , then at step 133 , the row number is incremented and step 130 is repeated fort the next row in the imaging array 100 . If, at step 132 , the last row of the imaging array has been clocked, then at step 134 , a row is selected for readout of accumulated voltages.
- each of the vertical column bus selection lines 110 a - 110 n is selected, either one at a time or all at once, such that the accumulated voltage is read out by the plurality of column bus amplifiers 112 a - 112 n.
- the processor 124 stores the selected output voltage in the memory 126 .
- the row number is incremented and step 134 - 138 are repeated for the next row in the imaging array 100 .
- step 140 If, at step 140 , the last row of the imaging array has been clocked, then at step 142 , if a desired flat field image has not yet been achieved, then at step 143 , the processor 124 recalls all of the stored output voltages and calculates individual DC gate voltage for each of the pixels 101 from the voltage values stored in the memory 126 that would make all of the pixels have the same gain. Steps 130 - 142 are repeated. If, at step 142 , the desired flat field image has been achieved, then at step 144 , the final values for the DC voltages to be applied to each of the pixels 101 are stored by the processor 124 in the memory 126 .
- the present invention has been described above in terms of adjusting DC gate voltages, the present invention is not limited to such an arrangement. In the more general case, all that is required is to adjust voltage difference between the DC gate and the high level of the EM clocked gate of an EM gain stage of a given pixel.
- the DC gate voltages can be applied one row at a time or on an individual pixel basis.
- multiple DC values for each pixel could be stored in the memory 126 to compensate for temperature fluctuations.
- multiple DC values can be stored in the memory 126 to adjust for different levels of average desired gain.
- the imaging array 100 can be run in a number of modes including snapshot mode and progressive scan mode. Table 1 details the clock rates that would be needed to implement the present invention for a 1024 ⁇ 1024 pixel array. The table assumes that only one row being clocked at a time with each pixel being clocked for 200 cycles to produce an overall gain of about 20 ⁇ . With an output source follower noise floor of ⁇ 10e RMS, the effective noise floor would be driven to ⁇ 1 e RMS.
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Abstract
Description
- The present invention relates generally to imaging systems, and more particularly to equalization of gain in the output of an array of imaging pixels which employ electron multiplication (impact ionization).
- A ubiquitous image sensor technology used in digital cameras is the charge-coupled device (CCD) imager. In a typical CCD imager, signal charge representative of incident radiation is accumulated in an array of pixels in an image area. Following an integration period, the signal charge is transferred to an output register by applying appropriate clocking or drive pulses to control electrodes. The signal charge is then read out from the output register and applied to a charge detection circuit to produce a voltage, which is representative of the amount of signal charge.
- It has been found that, with the application of proper gate potentials, a form of gain via impact ionization can be achieved in a CCD device. In a thesis entitled “Avalanche Gain In Charge Coupled Devices,” submitted to the Massachusetts Institute of Technology in August of 1986, Stephanie A. Gagar (hereinafter “Gager”) suggested the incorporation of an impact ionization multiplication of charge in a charge coupled device. Referring now to
FIG. 1A ,charge 2 is collected and accumulated under agate 4 in apotential well 6. The accumulatedcharge 2 is then transferred through anintermediate gate 8 to astorage gate 10 where it is temporarily stored. Theoriginal gate 4 wherein the charge was first accumulated is then biased into impact ionization. Referring now toFIG. 1B , charge is then transferred back from thetemporary holding gate 10 to the accumulatinggate 4 which is now biased as an impact ionization gate. This is accomplished by pulsing theholding gate 10 to a lower potential and transferring the charge through theintermediate gate 8 to the impact ionization region. For further gain, this procedure is repeated multiple times, i.e. 100 to 500 times, to build up charge. The gain per impact ionization transfer is roughly 1.015×. The gain after N impact ionization transfers is roughly (1.015)N. For N equal to 400, the resulting gain is about 386. Once sufficient charge has been built up, the charge is moved off of the CCD gates to a charge sensitive amplifier for charge-to-voltage conversion and read out. - A second design employing impact ionization can be found in pending commonly owned U.S. application Ser. No. 11/863,945 filed Sep. 28, 2007 to John Robertson Tower et al. (hereinafter “Tower et al.”), which is incorporated herein by reference in its entirety. Referring now to
FIGS. 2A and 2B , plan views of the Tower et al. EMCMOS device layout and architecture are depicted. Charge collection, storage, and electron multiplication (EM) regions are incorporated into asingle pixel 30 formed monolithically as an integrated circuit. Thepixel 30 includes electron multiplication (EM)gain regions EM gain register 36. InFIGS. 2A , 2B, there are a total of eight gates which constitute two stages of the EM gain register 36: PHI (clocked gate) 38,DC gate 40, EM (high voltage clocked gate) 42,DC gate 44, PHI 46, DC Gate 48, EM 50, and DC Gate 52. In the more general case, the Tower et al. device can comprise one or more impact ionization gain stages with implants to achieve charge transfer directionality. - A
readout structure 54 comprising a number of sub-structures are fabricated in thepixel 30 nested within theEM gain register 36. The light sensitive area, which creates electrons in proportion to the radiant energy incident on thepixel 30 can be an optical-to-charge conversion device such as a pinned photodiode (PPD) 56 as shown, a photogate, etc. ThePPD 56 is connected to and releases the accumulated charge to theEM gain register 36 by means of a PPD transfer gate (TR1) 58. A floatingdiffusion sense node 60 for receiving amplified charge from theEM gain register 36 and for converting the charge to a voltage is also connected to theEM gain register 36 by means of a floating diffusion transfer gate (TR2) 62. Thereadout circuitry 54 includes a row select gate 64, aPPD reset gate 66, asource follower transistor 68, and a sourcefollower reset gate 70. Power is supplied to thepixel 30 by means of power rail VDD 72. Thepixels 30 can be manufactured using a CMOS process, preferably a PPD CMOS process. - Referring now to
FIGS. 3A-3C , a schematic cross-section of one stage (4 gates) of the (EM)gain register 36 and accompanying applied potential diagrams of thepixel 30 is depicted. Each EM gain stage includes four gates: afirst DC gate 74, aphase 1clock gate 76, asecond DC gate 78, and aphase 2clock gate 80 which is employed to control the electron multiplication function. The clocking of theEM gain register 36 is done with two clock phases, as shown inFIG. 4A . During a first clock period shown inFIG. 3B , thephase 1clock gate 76 has a first potential applied to it to hold the accumulate charge (electrons) in acharge packet 82 in apotential well 84 below thephase 1clock gate 76. Meanwhile, a larger potential is applied to thephase 2clock gate 80 to create apotential well 86, which is initially devoid of charge.Barrier regions DC gates clock gates gain regions clock gates FIG. 3C , the potential of thephase 1clock gate 76 is changed to about 0 volts which is lower than the potential applied to theDC gates phase 1gate clock 76 now “spill” over into thewell 86 and can undergo impact ionization at theinterface 94 between thesecond DC gate 78 and thephase 2clock gate 80. As a result, theelectron charge packet 82 transfers to the well 86. Proper directionality is achieved by optimized implants. - The electrons in a
charge packet 82 are made to circulate a predetermined number of times through theEM gain regions pixel 30. Although the probability of impact ionization, and thus the mean gain per stage g for each of theEM gain regions charge packet 82 pass around the EM gain registers 36 and throughEM gain regions pixel 30, designated as N, can be high. The total gain of the cascaded multiplication elements (EM gain regions 32, 34) is given by M=gN. For N=600 and g=1.015 (1.5 percent probability of impact ionization), the total gain in the charge domain is over 7500×. -
FIGS. 4A-4D illustrate the operation of thepixel 30 ofFIGS. 2A and 2B . In a first time period (FIG. 4A ), acharge packet 82 accumulated in the pinned photodiode (PPD) 56 is loaded into theEM gain register 36 via the PPD transfer gate 58 (TR1). In a second time period (FIG. 4B ), thecharge packet 82 circulates around theEM gain register 36 through theEM gain regions charge packet 82 circulating aroundEM gain register 36 accumulates nominally about 400 transfers through theEM gain regions 32, 34 (200 cycles). In a third time period (FIG. 4C ), the amplifiedcharge packet 82 is transferred to the floatingdiffusion sense node 60 by means of the floatingdiffusion transfer gate 62 where the charge packet is converted to a voltage, the floatingdiffusion sense node 60 acting like a capacitor. In a fourth time period (FIG. 4D ), the floatingdiffusion sense node 60 and thePPD 56 are reset (global reset) via thePPD reset gate 66. Then the sequence is repeated for the next frame. - For both the “linear” architecture of the Gager device and the “circular” architecture of the Tower et al. device, as well as other CCD or CMOS pixels employing impact ionization in the prior art, if the pixels are arranged in a two dimensional array, a problem arises in that, since each pixel has slightly different design and process tolerances, the electron multiplication gain may differ from pixel to pixel. Although slight differences of device parameters may have little effect on the signal charge and output voltage for a single pass through EM gain regions, differences in gain are magnified as a result of charge circulating through the EM gain regions hundreds of times.
- Accordingly, what would be desirable, but has not yet been provided, is a means of equalizing gain among solid state EM gain pixels arranged in arrays.
- The above-described problems are addressed and a technical solution is achieved in the art by providing a method and apparatus for equalizing gain in an array of electron multiplication (EM) pixels. Each pixel comprises one or more impact ionization gain stages with implants to achieve charge transfer directionality. The invention is built upon the fact that the gain in the pixel architecture chosen is determined by the voltage difference between the DC gate and the high level of the EM clocked gate. A means is provided that permits the DC gate level to be adjusted individually when the selected row of pixels is being clocked to produce impact ionization gain.
- In the solution chosen, each pixel EM gain stage comprising a
phase 1 clocked gate, an EM clocked gate, and DC gates formed between thephase 1 clocked gates and the EM clocked gates, comprising the steps (a) applying initial voltages to each of the DC gates and the EM clocked gates of at least two pixels of a plurality of pixels; (b)clocking phase 1 clock gates and an EM clock gates associated with the at least two pixels of the plurality of pixels a predetermined number of times to achieve pixel intensity values after impact ionization gain; and (c) selectively adjusting the difference in voltage between the DC gate and corresponding EM clocked gate of the at least two pixels of the plurality of pixels until the difference between the resulting pixel intensity values and the average pixel intensity value needed to produce a desired image is below a predetermined threshold. The method can further comprise the step of, before step (b), exposing the plurality of pixels to light reflected from a flat field; wherein step (c) further comprises the steps of: (d) selectively adjusting voltages applied to the DC gates associated with the at least two pixels of the plurality of pixels such that substantially all of the pixels are projected to have the same gain; and (e) repeating steps (b) and (c) until the difference between the resulting pixel intensity values and the average pixel intensity value needed to produce a desired flat field image is below a predetermined threshold. - The array of EM pixels can be arranged in a 2-dimensions array of rows and columns, each row including a
common phase 1 clock gate bus and a common EM clock gate bus, each column including a common DC gate bus, and wherein step (c) further comprises the steps of (f) selecting a row of pixels; (g) applying a first set of DC voltages to the DC gate buses; (h) clocking aphase 1 clock gate bus and an EM clock gate bus associated with the row of pixels; and (i) holding thephase 1 clock gate buses and EM clock gate buses of the other rows of pixels at predetermined constant DC voltages so as to be inactive. Step (c) can further comprise the steps of (j) selecting another row of pixels; (k) applying a second set of DC voltages to each of the DC gate buses; (l) clocking aphase 1 clock gate bus and an EM clock gate bus associated with the another the row of pixels; (m) holding thephase 1 clock gate buses and EM clock gate buses of the other rows of pixels at predetermined constant DC voltages so as to be inactive; and (n) repeating steps (j)-(m) until substantially all rows in the plurality of pixels have achieved the same pixel intensity value after EM gain. - Each row can be associated with a row selection bus, wherein step (d) further comprises the steps of (o) selecting a row of pixels via the row selection bus; (p) reading a set of output voltage values of each of the columns; (q) storing the set of output voltage values in a memory; (r) selecting an additional row of pixels; (s) repeating steps (o)-(r) until substantially all of the rows in the plurality of pixels have achieved the substantially the same gain.
- The method can further comprise the steps of recalling all of the stored voltage values; performing a calculation to determine whether a flat field image has been achieved; calculating a plurality of DC voltages to be applied to corresponding ones of the DC gates in the array of pixels to achieve substantially the same gain; and storing values corresponding to the plurality of DC voltages in the memory.
- The present invention will be more readily understood from the detailed description of exemplary embodiments presented below considered in conjunction with the attached drawings, where like structures have like reference numerals, of which:
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FIGS. 1A and 1B depicts schematic cross-section of CCD gates and accompanying applied potential diagrams which illustrate the principle of impact ionization resulting in electron multiplication as outlined in the Gager paper of the prior art; -
FIGS. 2A and 2B are plan views of a Tower et al. EMCMOS pixel layout and architecture of the prior art; -
FIGS. 3A-3C depict a schematic cross-section of the electron multiplication gates and accompanying applied potential diagrams of the pixel ofFIGS. 2A and 2B ; -
FIGS. 4A-4D illustrate the operation of the pixel ofFIGS. 2A and 2B ; -
FIG. 5 is a block diagram of an arrangement of a two-dimensional array of electron multiplication (EM) pixels employing a calibration procedure according to an embodiment of the present invention; -
FIGS. 6A and 6B depict a timing diagram showing clocking conditions of the EM clock bus lines and thephase 1 clock bus lines of the imaging array ofFIG. 5 during calibration time; -
FIG. 7 is a block diagram of the two dimensional array of EM pixels ofFIG. 5 incorporated into a calibration fixture; and -
FIG. 8 is a flow chart illustrating steps of the calibration procedure of the present invention. - It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale.
- Referring again to
FIGS. 3A and 3C , the gain of anEM gain stage 36 of apixel 30 for the Tower et al. device is proportional to the electric field associated with thepotential well 86. The impact ionization gain is set by the difference between the voltage applied to the phase 2 (EM)clock gate 80 and the voltage applied to theDC gate 78. Thus, to match the gains across an array of pixels, the voltage differences between corresponding phase 2 (EM)clock gates 80 andDC gates 78 can be trimmed. While the voltage differences can be adjusted by trimming either or both of the voltages applied to the phase 2 (EM)clock gates 80 andDC gates 78, for practical reasons (such as clock settling and clock stability) it is easier to implement trimming of the voltages applied to theDC gates 78. While the voltages applied to thecolumn DC gates 78 are adjustable (typically ±100 mV), the voltages applied to thephase 1 clock gate 76 (typically about 4V-10V amplitude) and the phase 2 (EM) clock gates 80 (typically about 12V-23V amplitude) are common for all pixels actively being clocked. -
FIG. 5 shows an arrangement of a two-dimensional array 100 of electron multiplication (EM)pixels 101 employing a calibration procedure according to an embodiment of the present invention. AnEM pixel 101 is located at each of the intersections of a plurality of horizontal EM clock bus lines 102 a-102 n,phase 1 clock bus lines 104 a-104 n, and vertical DC gate bus lines 106 a-106 n. Each row oftie pixels 101 can have a common EM clock bus line, e.g., 102 a, (labeled N, N+1, N+2, etc.) andphase 1 clock bus, e.g., 104 a, (labeled N, N+1, N+2, etc.). Each column ofpixels 101 can have a common DC gate bus line, e.g., 104 a, (labeled (M, M+1, M+2, etc.). Each of the column DC gate bus lines 104 a-104 n can be independently controlled. If only one row (e.g., N) is clocked at one time, each of thepixels 101 in that row can have its gain adjusted pixel by pixel, by individual adjustment of the separate column DC gate bus lines 104 a-104 n. To read the output voltage produced by thearray 100 ofpixels 101, thepixels 101 are provided with horizontal row selection lines 108 a-108 n a vertical column bus selection lines 110 a-110 n, and a plurality of column bus amplifiers 112 a-112 n. - Referring now to
FIGS. 5 , 6A and 6B, a timing diagram shows clocking conditions of the EM clock bus lines 102 a-102 n and thephase 1 clock bus lines 104 a-104 n during calibration time (which can also mirror the relative clocking times during normal operation as shown inFIGS. 3B and 3C ). The voltage levels of an inactive row, e.g., row N+1, are held at constant DC levels such that no gain occurs in the pixels of that row. The EMclock bus line 102 b for row N+1 is held “low,” while thephase 1clock bus line 102 b for row N+1 is held “high.” For an active row, e.g., row N, during initial time interval T1 the EMclock bus line 102 a is also held “low,” while thephase 1clock bus line 104 a for row N is held “high.” During time interval T2, the EMclock bus line 102 a input is set to high, but because of capacitance a finite time is needed before the EM clock bus line voltage settles at about time t1. During the time interval T3, both thephase 1clock bus line 104 a is still held at a “high” level in order to allow for the voltage across the EMclock bus line 102 a to settle. Then at time t2, thephase 1clock bus line 104 a is set to “low,” which allows charge that has accumulated under aphase 1 gate to “spill” over into a charge well formed by the application of the high EM clock gate voltage during time interval T4 and thereby acquiring gain through impact ionization. At time t3, thephase 1 clock gate voltage is set back to high, but needs a finite amount of time to settle. Once thephase 1 clock voltage has settled at time t4, the EMclock bus line 102 a is set back to “low”, but takes a finite time to settle. - Referring now to
FIG. 7 a block diagram of the twodimensional array 100 ofEM pixels 101 incorporated into acalibration fixture 116 is depicted. The calibration fixture includes acamera 118 with focusingoptics 120, at least one aprocessor 124, and amemory 126. Theimaging array 100 and thecalibration fixture 116 can be incorporated entirely into a functioning camera or can be incorporated into stand-alone factory test equipment. - Referring now to
FIGS. 5 and 7 and the flow chart ofFIG. 8 , in operation, atstep 127, the vertical DC gate bus lines 106 a-106 n are set to a predetermined nominal DC voltage stored in thememory 126 for all of thepixels 101. Atstep 128, light 121 reflected from aflat field image 122 is focused by the focusingoptics 120 of thecamera 118 onto theimaging array 100. Atstep 130, a first active row is clocked for a predetermined number of clock cycles (to achieve a desired average pixel intensity value) with the waveforms discussed inFIG. 6 with respect to one of the EM clock bus lines 102 a-102 n and one of thephase 1 clock bus lines 104 a-104 n, while the others of the EM clock bus lines 102 a-102 n and thephase 1 clock bus lines 104 a-104 n are held at their predetermined constant values. Atstep 132, if this is not the last row in theimaging array 100, then atstep 133, the row number is incremented and step 130 is repeated fort the next row in theimaging array 100. If, atstep 132, the last row of the imaging array has been clocked, then atstep 134, a row is selected for readout of accumulated voltages. Atstep 136, each of the vertical column bus selection lines 110 a-110 n is selected, either one at a time or all at once, such that the accumulated voltage is read out by the plurality of column bus amplifiers 112 a-112 n. Atstep 138, theprocessor 124 stores the selected output voltage in thememory 126. Atstep 140, if this is not the last row in theimaging array 100, then atstep 141, the row number is incremented and step 134-138 are repeated for the next row in theimaging array 100. - If, at
step 140, the last row of the imaging array has been clocked, then atstep 142, if a desired flat field image has not yet been achieved, then atstep 143, theprocessor 124 recalls all of the stored output voltages and calculates individual DC gate voltage for each of thepixels 101 from the voltage values stored in thememory 126 that would make all of the pixels have the same gain. Steps 130-142 are repeated. If, atstep 142, the desired flat field image has been achieved, then atstep 144, the final values for the DC voltages to be applied to each of thepixels 101 are stored by theprocessor 124 in thememory 126. - Note that, although the present invention has been described above in terms of adjusting DC gate voltages, the present invention is not limited to such an arrangement. In the more general case, all that is required is to adjust voltage difference between the DC gate and the high level of the EM clocked gate of an EM gain stage of a given pixel.
- Optionally, the DC gate voltages can be applied one row at a time or on an individual pixel basis. In another embodiment, multiple DC values for each pixel could be stored in the
memory 126 to compensate for temperature fluctuations. In another embodiment, multiple DC values can be stored in thememory 126 to adjust for different levels of average desired gain. Theimaging array 100 can be run in a number of modes including snapshot mode and progressive scan mode. Table 1 details the clock rates that would be needed to implement the present invention for a 1024×1024 pixel array. The table assumes that only one row being clocked at a time with each pixel being clocked for 200 cycles to produce an overall gain of about 20×. With an output source follower noise floor of <10e RMS, the effective noise floor would be driven to <1 e RMS. -
TABLE 1 Example for 1024 × 1024 Imager Pixel Format 1024 × 1024 Frame Rate 30 Fps Vertical Blanking 300 μs Frame Readout 33 ms Row Time 32 μs EM Stages per Pixel 2 EM Gain per Stage 1.5% Total Gain per Frame 20× Number of EM Cycles Required 200 Addressing/Reading Row Time 12 μs EM Row Time Available 20 μs EM Clock Rate 10 MHz - It is to be understood that the exemplary embodiments are merely illustrative of the invention and that many variations of the above-described embodiments may be devised by one skilled in the art without departing from the scope of the invention. It is therefore intended that all such variations be included within the scope of the following claims and their equivalents.
Claims (24)
Priority Applications (5)
Application Number | Priority Date | Filing Date | Title |
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US12/128,890 US7948536B2 (en) | 2008-05-29 | 2008-05-29 | Gain matching for electron multiplication imager |
EP08874511A EP2279613A4 (en) | 2008-05-29 | 2008-10-27 | Gain matching for electron multiplication imager |
JP2011511585A JP2011522482A (en) | 2008-05-29 | 2008-10-27 | Gain matching for electron multiplier image sensors |
PCT/US2008/081284 WO2009145803A1 (en) | 2008-05-29 | 2008-10-27 | Gain matching for electron multiplication imager |
TW097144541A TW200950508A (en) | 2008-05-29 | 2008-11-18 | Gain matching for electron multiplication imager |
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US12/128,890 US7948536B2 (en) | 2008-05-29 | 2008-05-29 | Gain matching for electron multiplication imager |
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US20090295952A1 true US20090295952A1 (en) | 2009-12-03 |
US7948536B2 US7948536B2 (en) | 2011-05-24 |
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US12/128,890 Active 2029-12-04 US7948536B2 (en) | 2008-05-29 | 2008-05-29 | Gain matching for electron multiplication imager |
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US (1) | US7948536B2 (en) |
EP (1) | EP2279613A4 (en) |
JP (1) | JP2011522482A (en) |
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WO (1) | WO2009145803A1 (en) |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20090059049A1 (en) * | 2007-08-28 | 2009-03-05 | Sanyo Electric Co., Ltd. | Image pickup apparatus |
AU2013287623B2 (en) * | 2012-07-10 | 2015-09-24 | Ricoh Company, Ltd. | System including operation device and information storing apparatus, method performed by the system, and the information storing apparatus |
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CN112269075B (en) * | 2020-09-25 | 2022-06-24 | 华东光电集成器件研究所 | EMCCD charge transfer efficiency testing method |
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JP4835836B2 (en) * | 2006-03-30 | 2011-12-14 | 日本電気株式会社 | Electron multiplication gain calibration mechanism and electron multiplication gain calibration method |
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- 2008-05-29 US US12/128,890 patent/US7948536B2/en active Active
- 2008-10-27 JP JP2011511585A patent/JP2011522482A/en active Pending
- 2008-10-27 WO PCT/US2008/081284 patent/WO2009145803A1/en active Application Filing
- 2008-10-27 EP EP08874511A patent/EP2279613A4/en not_active Withdrawn
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Also Published As
Publication number | Publication date |
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WO2009145803A1 (en) | 2009-12-03 |
US7948536B2 (en) | 2011-05-24 |
EP2279613A4 (en) | 2011-09-07 |
TW200950508A (en) | 2009-12-01 |
EP2279613A1 (en) | 2011-02-02 |
JP2011522482A (en) | 2011-07-28 |
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